1. Field of the Invention
This invention relates to a semiconductor device provided with a semiconductor module that is mounted on a mobile object such as a vehicle, the invention contributing particularly to a reduction in influence of a thermal stress due to heating.
2. Description of the Background Art
Conventional semiconductor device is generally provided with a semiconductor module having a semiconductor element sealed with a resin and a screw through-hole provided at a central part thereof, a plate-like presser spring disposed at one side of the semiconductor module, a reinforcing beam for reinforcing the presser plate-like spring, and a heatsink disposed at the other side of the semiconductor module, wherein the semiconductor module is fixed to the heatsink by a screw inserted from the reinforcing beam side into the screw through-hole of the semiconductor module via the reinforcing beam and the presser plate-like spring, and the presser plate-like spring has a slit formed for dividing a peripheral part thereof (see Patent Publication 1, for example). Patent Publication 1: JP-A—2005-235992 (paragraph [0005], FIG. 1, etc.)
The conventional semiconductor device is formed by fixing the members such as the semiconductor module, the plate-like spring, and the reinforcing beam to the heatsink. The heat generated in the semiconductor module is discharged to the heatsink, and the heat transferred to the heatsink is discharged to the outside via a cooling member attached to a reverse surface (a surface reverse to a surface on which the semiconductor module is attached).
With such structure, a thermally conductive member in the form of a paste or a flexible sheet is generally provided between the semiconductor module and the heatsink and between the heatsink and the cooling member for suppressing heat resistance. In order to realize effective thermal conduction by the thermally conductive member, it is necessary to reduce a thickness of the thermally conductive member disposed between the heatsink and the semiconductor module and between the heatsink and the cooling member by increasing a pressure contact force between the members as well as to increase flatness of the heatsink for maintaining uniformity of the thickness. Therefore, a member for forming the heatsink is required to have high rigidity capable of enduring the pressure and ensuring the flatness.
However, since a copper that is generally used as a material satisfying the conditions has a high specific gravity and is expensive, there has been a problem of difficulty in achieving downsizing, light weight, and low cost of the semiconductor device when the copper is used as the member for the heatsink or the rigidity is increased by increasing the thickness of the member. Also, since a plate material that is easily available is generally used in the case of using the copper, there has been a problem of difficulty in shaping from the reasons such as cost suppression in mechanical processing and the like.
Further, there has been a problem that it is difficult to achieve a reduced and uniform thickness of the thermally conductive member due to reduction of rigidity of the heatsink and deformation of the heatsink that is caused when the heatsink is reduced in thickness, weight, and so forth in order to achieve downsizing, light weight, and reduced cost of the semiconductor device. Also, there has been a problem that the semiconductor device has difficulty in stably exhibiting the original performance since the deformation of the heatsink is subject to various factors such as influences of the pressing force by the reinforcing beam and the plate-like spring, influence of the pressure contact force for keeping the thermally conductive member thin and uniform, and influence of the thermal stress caused by a use environment temperature of the semiconductor device and the heat generation in the semiconductor module.